Severe asthma is a significant health care concern with patients having poorer asthma control, frequent asthma attacks, and increased heath care costs compared to milder asthma phenotypes. Boys have an increased prevalence of severe asthma compared to girls. This increased prevalence of severe asthma switches from boys to women around puberty, suggesting a role for sex hormones. However, it remains unknown how gender and ovarian hormones affect severe asthma pathogenesis. IL-17A, a cytokine secreted by CD4+ T cells and ?? T cells, is increased in the bronchoalveolar lavage fluid of patients with severe asthma compared to milder asthma phenotypes. IL-17A is also associated with increased airway inflammation, airway responsiveness (AR), and mucus production. Our preliminary data show increased IL-17A protein expression in Th17 differentiated cells from women compared to men. We also show 17?-estradiol (E2) and progesterone (P4) administration in ovariectomized female mice increased Th17 differentiated cell expression of IL-17A. We hypothesize that female gender, via ovarian hormones, increases Th17 cell differentiation and IL-17A secretion from Th17 cells and ?? T cells. We further hypothesize that in vivo 17?-E2 and P4 administration increases Th17-mediated airway inflammation, AR, and mucus production in mice. In Aim 1, we will differentiate Th17 cells from healthy women and men and determine the role of gender in Let-7f regulation of IL-23 receptor (R) and IL-17A protein expression on Th17 differentiating cells. In Aim 2, memory T cells will be isolated to determine if women with severe asthma have increased IL-17A secreting memory CD4+ T cells compared to men with severe asthma. In Aim 3, we will delineate the mechanisms by which in vivo administration of 17?-E2 and P4 increases IL-17A-mediated airway inflammation in mice. Our results will be the first to determine the role of gender and ovarian hormones on IL-17A-mediated airway inflammation in severe asthma. Our findings may also be helpful in designing future clinical trials by determining if women and men with severe asthma have differential IL-17A production and responses to therapeutics in clinical trials for moderate to severe asthma.